Illumination device

ABSTRACT

An embodiment of an illumination device comprising a broad band artificial light source and a non-liquid chromatic diffuser transparent to visible light comprising a dispersion of elements of nanometrical dimensions of a first material with of certain refractive index in a second material with different refractive index, wherein the light is scattered producing a separation and different distribution between cold and hot components of the light originally produced by the source, according to a scattering process in “Rayleigh” regime. The device allows illumination effects similar to those of natural outdoor environments to be reproduced in indoor environments.

PRIORITY CLAIM

The present application is a Continuation of copending U.S. patentapplication Ser. No. 13/001,050, filed Apr. 27, 2011; which applicationis a national phase application filed pursuant to 35 USC §371 ofInternational Patent Application Serial No. PCT/EP2009/057672, filedJun. 19, 2009; which further claims the benefit of Italian PatentApplication MI2008A001135, filed Jun. 24, 2008; all of the foregoingapplications are incorporated herein by reference in their entireties.

RELATED APPLICATION DATA

This application is related to U.S. patent application Ser. No.12/977,070, filed 23 Dec. 2010, now U.S. Pat. No. 8,469,550 entitledOPTICAL SKY-SUN DIFFUSER issued on 25 Jun. 2013, and which isincorporated herein by reference in its entirety

TECHNICAL FIELD

An embodiment relates to an illumination device. More in particular, anembodiment relates to an artificial illumination device capable ofreproducing in indoor environments the light and colors of the sun andof the sky, by combining a broad-band artificial light source and aparticular nanostructured chromatic diffuser.

An embodiment produces an innovative type of artificial illuminationcapable of reproducing a fundamental and to date neglected aspect ofnatural illumination, that is, the simultaneous presence of twodifferent light sources, namely skylight and sunlight, which differ incolor, intensity, direction and spatial extension. In fact, the sky isresponsible for the presence of a scattered light with blue as dominantcomponent, i.e. “cold” in common terms, emitted from an extended surfaceand therefore capable of illuminating shadows. Instead, the sun isresponsible for the presence of a light with a limited blue component,i.e. “warm”, which being emitted from an area subtending a limited solidangle, illuminates the objects only with direct light.

BACKGROUND

Improvement in the quality of artificial illumination is today a toppriority requirement. In fact, there are increasingly more circumstancesin which man finds himself spending a large part of his life inartificial illumination conditions. This is due to the constructionalfeatures of many industrial spaces, hospitals, department stores,underground railways, airports and the like, whose indoor areas are notexposed to direct skylight and sunlight. Moreover, in various regions ofthe planet, the conditions of low temperature (for example in Canada)or, vice versa, high temperature and humidity (for example inSingapore), which characterize lengthy periods of the year, encouragemore and more development of underground urban planning, as it is mucheasier to achieve satisfactory climate control underground. Finally, thequality of artificial illumination has a considerable impact on thequality of life for populations living at high latitudes, where there islittle or no sunlight for lengthy periods of the year.

On the other hand, the energy question today places the need to reducepower consumption used for illumination at the forefront. As can be seenfrom recent legislation, this need provides for the elimination, withina few years, of conventional incandescent lighting, which produces ablack body emission spectrum similar to sunlight, but which dissipatesmost of the energy in heat, in favor of new technologies, such as LEDsand laser diodes. LED technology, already widely used for backlightingscreens and panels, in road signs and in motor vehicles, is todaypreparing to enter the market of indoor and outdoor lighting. One of themain difficulties is in this case constituted by the quality of thelighting, above all for low cost types of sources, which exhibit lowerconsumption. This is the case, for example of InGaN—GaN LEDs emitting inthe blue region (at 430-470 nm) completed by the presence of a phosphorwhich emits broad-band radiation in the yellow region (around 580 nm).These sources have a spectral profile differing substantially from thatof a black body, presenting a peak of maximum intensity at the emissionwavelength of the LED, and a second peak of lesser intensity at themaximum phosphor emission efficiency. The difficulty linked to this typeof source is related both to the very high color temperature (≈7000K),as described in U.S. Pat. No. 7,259,400 which is incorporated byreference, which gives the light the characteristic bluish color, and tothe lack of green and red components in the spectrum. Although this lackis not noticed when illuminating a white object, given that the yellowcomponent produced by the phosphor excites, in a balanced manner, thecones in the eye sensitive to red and green, it becomes important whenilluminating colored environments, given that green or red objectsappear dark.

Within the scope of current technological development, the majority ofefforts aimed at improving the quality of illumination are concentratedon the spectral characteristics of the light produced having the objectof making it perceived to be as close as possible to sunlight. Withinthe context of the definition above, this approach nonetheless does notinclude the aforesaid fundamental aspect characterizing naturalillumination, namely the presence in nature of not one but two differentlight sources: the sky and the sun. The effect can be understood byconsidering the different CCT (Correlated Color Temperature) of the twosources, defined as Planck radiator temperature (black body radiation),which is perceived by the eye as a color closer to that of the source inquestion. If we consider, for example, natural illumination in the lateafternoon, when the sky, almost as luminous as the sun, has a CCT ofover 9000K, and the sun has a CCT of under 4000K, it is evident that thespectrum resulting from the sum of the two sources is nowhere near ablack body spectrum. Nonetheless, this type of illumination is extremelypleasing to the eye. Therefore, the presence of this particulardichromatism, associated with direct and scattered light, is animportant element, not considered previously, to be added to theprevious ones in order to assess the quality and pleasantness ofartificially created illumination.

It is also important to note that an illumination method based on asingle type of source can at the most simulate, in the case of aspectral profile similar to that of the sun, “lunar” illumination. Inthis context, as the shadows are very dark, they are not pleasing. Forthis reason, artificial illumination often uses many sources, orreflections on walls or ceiling, to minimize shadows.

A first proposal for artificial illumination based on indoorreconstruction of natural illumination as composed of sky and sun waspresented in a work exhibited in various science and art exhibitions,also presented at the Genoa Science Festival in 2003 and 2005 and inVilnius Railway Station (Lithuania) in 2007 (www.diluceinluce.eu, whichis incorporated by reference). In these contexts various installationsand various experimental apparatus were produced, including “indoor”reconstruction of the sky, i.e. reconstruction of the Rayleighscattering process caused by nanometrical density fluctuations of atransparent medium which, in the case of the atmosphere, determines thelight and color of the sky and of the sun. As scattering medium anaqueous dispersion of silica nanoparticles, with diameter of around 20nm, was used. This dispersion, presenting refractive index fluctuationsof considerable amplitude (approximately 15%) on scale lengths below1/10 of the wavelength, allowed the production of a good diffuseroperating in Rayleigh regime. At the maximum concentrations used, i.e.for a silica volume fraction of 2% of the dispersion volume, it provedcapable of producing, on a beam of light passing through it for a fewmeters, the same color variation which, in the atmosphere, requireshundreds of kilometers of distance. The dispersion thus produced wasplaced in transparent PMMA containers for containment. “White” lightsources were then used to simulate the sun, namely halogen lamps withcalibration filters or mercury vapor discharge lamps. By using differentdispersion concentrations, different sky volumes, different installationgeometries, comprising combinations of containers of different shape anddimension, different positions of sky and sun, and the presence ofabsorbent or reflective screens to simulate clouds, spectacularreconstructions were obtained of light effects due to the presence ofthe sky and the sun at different times of day.

However, the dispersion of nanoparticles in water presents numerousproblems that make its use in the lighting sphere almost impossible. Infact, due to the different specific weight P_(s), between water andnanoparticles, which typically increases with the refractive index valueof the nanoparticles, n₁, (e.g. P_(s)=2.2 g/cm³ and n₁=1.5 for SiO₂,while P_(s)=4.23 g/cm³ and n₁=2.7 for TiO₂), the nanoparticles tend todeposit through gravity on the bottom of the container, as they are heldin suspension only through Brownian motion. For this reason thesuspension must be stirred periodically. For the same reason theconcentration of nanoparticles is not constant, but decreases withheight. The problem may be reduced, but not eliminated, usingnanoparticles of extremely small diameter, in order to maximize theeffect of Brownian motion. In this case, however, the scatteringefficiency is influenced negatively, a fact that implies the use ofdiffusers of very high depths (at least in the order of tens of cm).Moreover, the suspension in liquid is somewhat unstable from abacteriological viewpoint, especially if continuously exposed to light.It then presents the risk of freezing, which prevents its use foroutdoor installations. Moreover, the liquid medium presents the problemof containment, which is important in the case of diffusers ofmedium-large dimensions, and the need to combat pressure due to theheight of the liquid, which implies the use of containers produced inthick material (several cm) in the case of heights of over one linearmeter.

U.S. Pat. No. 6,791,259 B1, which is incorporated by reference,describes a white light illumination system comprising an LED or laserdiode, a light diffuser material and a phosphor or luminescent dyematerial. The diffuser material preferably comprises particles dispersedin a substrate. The particles that scatter light have a diameter between50 and 500 nm, preferably a diameter between λ/3 and λ/2, where λ is thewavelength of the emission peak of the radiation source. In thisapplication, however, the color nanodiffuser is integrated at the levelof the active element of the source, that is, it is positioned eitherbefore the phosphor or in the phosphor, in order to scatter preferablythe blue component produced by the LED or laser diode, otherwise withlow divergence, and to uniform it with the yellow component scattered bythe phosphor, instead produced with a wide angle of divergence. The factthat the two yellow and blue components are scattered from practicallycoincident diffuser centers is a necessary condition to remove the“halo” phenomenon, characterized by the presence of a dominant bluecolor in the direction of maximum emission, and of a dominant yellowcolor in the peripheral area of the light cone produced by the source,that is, to uniform color distribution of the radiation at differentangles.

WO 02/089175, which is incorporated by reference, describes lightsources based on UV with reduced dispersion of UV radiation. The lightsources are LEDs which emit in the UV and which are combined with UVreflectors constituted by particles dispersed in a solid materialtransparent to visible light. A phosphorescent material is applied tothe UV source to convert UV radiation into visible light. In aparticular embodiment the phosphorescent material is applied to thesurface of the UV LED and a layer of diffuser material is applied to thephosphorescent layer. The aim of this illumination device structure isto reduce the amount of UV radiation not converted into visible lightand does not tackle the problem of reproducing a light similar tonatural light produced by the sun and the sky.

SUMMARY

An embodiment, in its numerous variants, produces a new type ofartificial illumination device capable of reproducing the simultaneouspresence of two different color components: skylight, in which blue(cold) is dominant, and sunlight, with a low blue component (“warm”),which illuminates objects with direct light.

Therefore, while the aim of known illuminating devices is to produceuniform white light, a result at times obtained through appropriate andhomogeneous mixing of sources of different color, an embodiment proposesthe opposite object of “separating” different color components of asource with broad spectral bandwidth. However, this color separationdoes not take place generically, as, for example, through the use of arefractor (such as a prism) which diverts different wavelengths todifferent angles, or of a filter that absorbs a portion of the spectrumof the source and transmits the complementary portion, or of a mirrorthat reflects and transmits other portions. On the contrary, it takesplace as a result of the same mechanism that gives rise to colorseparation in nature, creating the correct spectral distributioncharacteristic of skylight and sunlight.

An embodiment is an illumination device comprising a broad-bandartificial light source comprising one or more active elements whichemit photons or absorb them and then re-emit them at a higherwavelength, and a chromatic diffuser located downstream thereof andcomprising elements of a first non-liquid material transparent tovisible light and having refractive index n₁ dispersed in a secondnon-liquid material transparent to visible light and having refractiveindex n₂, whereby |n₂/n₁−1|>0.1 and whereby the typical lineardimension, d, of the dispersed elements of the first material satisfythe condition 5 nm≦d≦300 nm, characterized in that the maximum dimension(L_(max)) of said diffuser is equal to or greater than double theminimum dimension (d_(min)) of the projection of the largest activeelement belonging to said light source in the plane perpendicular to:

-   -   a. in the case of light source with anisotropic emission, the        maximum emission direction (I_(max)) of said source; or    -   b. in the case of light source with isotropic emission, the        direction defined by the straight line that joins the two        closest points (I_(prox)) of said source and of said diffuser,        as expressed by the relation L_(max)≧2 d_(min);        wherein the maximum dimension L_(max) of said diffuser is        defined as the greater of the maximum distances of two points        belonging to the projection of the diffuser in a plane        perpendicular or parallel to the directions I_(max) or I_(prox)        defined above.

In the case in which L_(max) is the distance between two pointsbelonging to the projection of the diffuser in a plane perpendicular tothe directions I_(max) or I_(prox), it is also defined as “transversedimension” of the diffuser, while in the case in which L_(max) is thedistance between two points belonging to the projection of the diffuserin a plane parallel to the directions I_(max) or I_(prox), it is alsodefined as “longitudinal dimension” of the diffuser.

The phenomenon according to which an embodiment of the device proposedis capable of producing separation and different distribution between“cold” and “warm” components of the light originally produced by thesource is the light scattering process in nanostructured transparentmaterials in “Rayleigh” regime, that is, in conditions in which theincrease of scattering efficiency is inversely proportional to thefourth power of the wavelength. This phenomenon is the same one thatdetermines, in nature, the color and luminosity of the sky, the color ofshadows as areas illuminated by light coming from the sky, the color ofdirect sunlight after it has passed through the atmosphere, and itsvariations according to time of day, seasons, etc.

When a beam of collimated light travels through empty space, or aperfectly transparent, i.e. which does not absorb in the spectral regionof interest, and homogeneous medium, it propagates undisturbed, so thatthe eye can only perceive its presence if it is directly in its path.However, if the transparent medium is not homogeneous, so that itexhibits spatial fluctuations of refractive index, then the path of thebeam is “disturbed”, and part of the light is diverted from its originaltrajectory. The method according to which the phenomenon appears dependson the dimensions and forms of these fluctuations.

(i) If the dimensions of the heterogeneities are very large with respectto the wavelength, “refraction” is obtained.

(ii) If the dimensions of the heterogeneities are only slightly largerwith respect to the wavelength, “diffraction” is instead obtained.

(iii) If the dimensions of the heterogeneities are comparable orslightly lower with respect to the wavelength, the light is scattered ina substantially homogeneous manner with respect to its spectralcomponents, at least for angularly integrated radiation, and it isscattered principally at small angles with respect to the incidentdirection. The effect, often called Mie scattering, is that produced byfog and mist.

(iv) Finally, if the dimensions of the non-homogeneities are muchsmaller than the wavelength of the light, the phenomenon on which anembodiment is based, that is, “Rayleigh scattering”, is obtained. Thisis a process according to which unpolarized light is scatteredsubstantially in all directions with maximum amplitude at 0 and 180degrees with respect to the incident direction, and with an increase inefficiency inversely proportional to the fourth power of the wavelength.

The most spectacular example of “Rayleigh” scattering in nature isscattering that occurs when sunlight passes through the atmosphere. Inthis case scattering is produced by refractive index fluctuationsassociated with density fluctuations of the gases forming thisatmosphere. These fluctuations occur at much shorter scale-lengths thanthe wavelength of sunlight and, especially in the case of the highatmosphere, are spaced apart by a large distance with respect to thescale-length of these fluctuations, and therefore arranged randomly. Asa result of the scattering process, the component of solar radiationwith the shortest wavelength (that is, blue, given that the nearultraviolet, with even shorter wavelength, is substantially absorbed andbarely perceived due to low sensitivity of the human eye in thisspectral region) is the one most scattered. The blue component (at 430nm), is in fact scattered about six times more than the red component(at 670 nm). This important difference in the scattering efficiency isthe principal reason determining the blue color of the sky.

To better understand the nature and importance of the light scatteringprocess in the sky, whose indoor reconstruction represents part of anembodiment, it is useful to recall stock photographs describing whathappens on the moon, where there is no atmosphere. The sky is black onthe moon, even when the sun is at midday. The lunar environment isilluminated by a single source, the sun, whose light is and remainswhite at all times of the day. The shadow areas, illuminated only by thelight reflected by surrounding objects, are almost as dark during theday as at night. Instead, on Earth the environment is illuminated notonly by the sun but also by the sky. Direct sunlight, when it has noscattered blue component, has a color varying from pale yellow to orangeto red, depending on the different depth of atmosphere crossed in thedifferent hours of the day. The “warm” color of direct sunlight is notedparticularly when it illuminates an object through a narrow window,whose limited aperture exposes it entirely to sunlight but shields itfrom the majority of skylight. The sky, in turn, illuminates the scenewith blue. The effect can be noted by looking carefully at shadows. OnEarth “in shadow” does not mean “in the dark”, as it does on the moon,but in skylight.

Outdoors, with the sun at the zenith, in total absence of reflections,shadows, windows etc., the skylight is summed with the sunlight,obtaining a light not dissimilar to the light that would be obtained inthe absence of atmosphere. However, the shadow, even only that of theperson observing, is always present, and perceived as more luminous themore the sun is low on the sky.

In order to define the characteristic parameters of an embodiment, it isadvisable to take as reference the intervals which in naturecharacterize (i) the ratio between the power of sunlight and skylightand (ii) the color-temperature values of the two different sources,during different hours of the day, season, etc. In a typical spring day,in the early afternoon, with clear skies and at a subalpine latitude inItaly, it has been measured that the sky contributes for about 20% tothe total luminosity, produced by sky+sun. Measurement was performed bycomparing the power of the radiation striking an object in the shadow ofa screen covering the sun but not the sky, with the power striking thesame object without the screen. Assuming that the radiation scattered bythe atmosphere towards Earth has the same power as that scatteredtowards the outside of the atmosphere, and ignoring the effect of theEarth's curvature, it was obtained that the sky scatters incidentradiation with a scattering efficiency η=⅓, where η is the fraction ofincident light power scattered by the entire depth of the sky. Slightlylower values are obtained when the sun is at the zenith. Higher valuesare instead obtained in the early morning or in the evening. One hourbefore sunset, in the same place and season, a scattering efficiency ofη=⅔ was measured. Finally, it is noted that at dawn or dusk almost allthe incident light is scattered (η=1).

With regard to color-temperature assessment of the two sources, it maybe advisable to refer to the CCT (Correlated Color Temperature), whichis the Planck radiator temperature (black body radiation) perceived bythe eye as a color closer to that of the source in question. The CCTrelative to the resultant of sky and sun at midday on a clear day isaround 5500K, that is, a value of about 10% less than the CCT of solarradiation outside the atmosphere (the difference is due at least partlyto the blue radiation scattered outside). The light produced by theclear sky has a CCT typically of between 9000 and 12000K, with muchhigher values in the early morning or late evening. With regard to theCCT of direct solar radiation it can reach 5200K in conditions with thesun at the zenith, but typically varies between 4700K (at midday) and3500K (one hour after dawn or before dusk). Even lower CCT values areobtained at sunrise and sunset, through the joint effect of Rayleighscattering and atmospheric refraction.

In the context of an embodiment, the concept of shadow is revolutionizedgiven that, just as outdoors, in shadow does not mean in the dark, butin skylight. As this approach is capable of reconstructing the colorcontrast between light and shadow, it allows the particular beautycharacterizing the nature of daylight on Earth to be reproduced inindoor environments.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be illustrated with reference to theaccompanying figures, wherein:

FIG. 1 is a diagram illustrating the definition of spectral bandwidth ofa light source according to an embodiment;

FIGS. 2A, 2B and 3 illustrate the definition of light source dimensionaccording to an embodiment;

FIG. 4 illustrates the definition of dimension of the diffuser accordingto an embodiment;

FIGS. 5A-5E schematically illustrate a first embodiment of theillumination device;

FIG. 6 schematically illustrates a second embodiment of the illuminationdevice;

FIG. 7 schematically illustrates the type of illumination produced bythe embodiment of the illumination device according to FIG. 6;

FIG. 8 schematically illustrates a third embodiment of the illuminationdevice;

FIG. 9 schematically illustrates the type of illumination produced by anillumination device according to prior art;

FIG. 10 schematically illustrates the type of illumination produced byan illumination device according to an embodiment in the form describedin FIGS. 6 and 8;

FIG. 11 schematically illustrates a fourth embodiment of theillumination device;

FIGS. 12A, 12B, 12C schematically illustrate a fifth embodiment of theillumination device;

FIGS. 13A, 13B, schematically illustrate a sixth embodiment of theillumination device;

FIGS. 14A, 14B, 14C, 14D, 14E, 14F schematically illustrate furtherembodiments of the illumination device;

FIG. 15 shows an embodiment in which the light source and the chromaticdiffuser are totally separated and spaced apart; and

FIG. 16 is a diagram relative to the spectral characteristics of theradiation scattered and transmitted by a “nanogel” diffuser materialemployed according to an embodiment.

FIG. 17 is a diagram relative to the spectral characteristics of theradiation scattered and transmitted by a PMMA/TiO₂ nanocompositediffuser employed according to an embodiment.

FIG. 18 shows an embodiment of a device in an “after sunset”configuration.

DETAILED DESCRIPTION

An illumination device is produced in two different principalembodiments, which are not alternative but may also be combined:

(i) The first, hereinafter referred to as “midday”, has the object ofreproducing the natural illumination condition typical of the middlehours of the day. It is characterized by the presence of two components,sky and sun, of different color but each of which is of unequivocallydefined color. In fact, at midday the sun is whitish-yellow, and the skyhas the same blue color more or less all over. Moreover, at midday thesun dominates the scene, being directly visible from most of it. If anarea is exposed only to sunlight, or skylight, or both, it will beilluminated with yellowish, blue or white light.

(ii) The second, hereinafter referred to as “sunset”, has the object ofreproducing the natural illumination condition typical of evening. Inthis case the sky does not have the same color all over, due to the longpath sunlight has to travel through it, which allows scattering not onlyof blue but also of the other colors. Moreover, in the evening the sunis not necessarily directly visible, as it is low on the horizon andtherefore often blocked by a mountain or building, or by far off clouds.Its presence is perceived as a result of “reflection” or “scattering” ofits direct light by clouds, or by mountaintops.

(iii) The third, hereinafter referred to as “after sunset”, has theobject of reproducing the natural illumination condition typical of whenthe sun has disappeared beneath the horizon. In this case the only colorthat remains to tinge the sky and the scene is the blue.

The “midday” embodiment uses a diffuser whose maximum dimension L_(max)is the “transverse dimension”, defined as maximum distance between twopoints belonging to the projection of the diffuser in the planeperpendicular to the maximum emission direction I_(max) or the directiondefined by the straight line joining the two closest points (I_(prox))of source and diffuser. In fact, only this condition allows theproduction of an extended element, which emits scattered light ofuniform blue color, capable of coloring shadows. It also allows directvision of the source from the rest of the scene. Just as in nature, whenit has no blue component, direct light assumes a yellowish color.

The “midday” configuration has two different variants.

i) A first “panel” variant with the aim of obtaining illumination by thediffuser of shadows created by the presence of specific objects presenton the scene. For this purpose the transverse dimension of the diffuseris large not only with respect to that of the source, but also withrespect to that of the objects whose shadow is to be illuminated. It isnoted that shadows are portions of the scene from which direct vision ofthe source (e.g. the sun in nature) is covered by the presence of anobstacle, but not the total or partial vision of the diffuser (e.g. ofthe sky). Due to the large dimension of the diffuser, the areas on thescene for which direct illumination by the source is prevented by anobstacle can in any case be exposed to the light generated by thediffuser, which therefore colors the shadows thereof.

It is noted that for the effect to be produced the transverse dimensionof the diffuser is large with respect to that of the source (in at leasta transverse direction), in order to obtain shadow areas illuminated bythe diffuser but not directly by the source. It is also noted that thediffuser is a separate element from the source, from which it may beseparated by any arbitrary large distance. Indeed, ideal simulations ofthe natural illumination by direct and diffused light from the sun andthe sky would require the source to be placed extremely far from thediffuser in order to guarantee all shadows to have the same orientation.However practical realizations might benefit from closer distances inorder to optimize the overall efficiency of the device.

ii) A second variant of this type, hereinafter referred to as “Spot”,with the aim of obtaining illumination by the diffuser of portions ofthe scene outside the light cone that the source would be capable ofemitting in the absence of the diffuser. This objective is achievedusing a source that generates, in the absence of the diffuser, a lightcone with limited aperture and a diffuser with arbitrarily smalltransverse dimension with respect to the dimension of the environment,provided it is always greater than double that of the source. In thisconfiguration, the “Spot” source generates two light cones, of differentaperture, different intensity (greater on the inside and lesser on theoutside) and different color temperature, “warmer” on the inside and“colder” on the outside. The sharper the transition between light anddark in the absence of diffuser, the sharper the transition in intensityand color temperature between the two light cones will be. The effectthat this device creates on the scene, with regard to the difference inillumination and coloration intensity produced by the two light cones,is similar to the illumination effect produced by the sun and by thesky. The inner cone (“warm”) illuminates the scene as occurs in naturefor an object exposed to the sum of afternoon sunlight and skylight. Theouter cone (“cold”) illuminates the scene as occurs in nature for anobject in shadow, exposed only to skylight. In this manner, these areasappear to the view as areas “in shadow” even if the actual object (e.g.the obstacle) that generates this shadow is missing on the scene. Unlikethe first “panel” variant, the “Spot” source, is not capable ofilluminating the shadows of real objects present on the scene.

It should be noted that for the effect to be produced it is necessaryalso in this case for the diffuser to have a large transverse dimensionwith respect to the source (in at least a transverse direction), inorder to obtain transmitted and scattered light cones of differentaperture. This result may be obtained, for example, by placing theextended diffuser immediately downstream of a collecting lens capable ofcollimating direct light inside the cone in question, said lens in orderto operate correctly must have larger transverse directions to those ofsaid source. Alternatively, the diffuser element may be incorporated inthis array of optical elements.

On the contrary, the “sunset” embodiment uses a diffuser whose maximumdimension L_(max) is the longitudinal one, defined as the maximumdistance between two points belonging to the projection of the diffuserin a plane parallel to the directions I_(max) or I_(prox) defined above.In fact, only in this case may luminous radiation be transmitted in thediffuser for a path considerably greater than the width of the diffuserin the transverse plane, as moreover occurs in nature in the evening. Infact, at sunset (and also at sunrise) the depth of atmosphere passedthrough by sunlight, with the sun low on the horizon, is a few hundredsof km, therefore much greater than the height of the atmosphere, in theorder of tens of km. This geometrical condition allows all the spectralcomponents of light of the source, starting from blue, then green,yellow, orange, etc., to be effectively scattered by the “sky”, untilonly the red component, which represents direct sunlight at sunset,reaches far off objects.

Unlike the “midday” condition, in which the observer looks at thediffuser panel according to a direction in the region of I_(max) orI_(prox), in the “sunset” configuration the observer looks at thediffuser from a perpendicular direction.

It is noted that in order for different portions of the diffuser to emitlight of different color towards the observer, the diffuser has a largelongitudinal dimension, and in particular greater than (at least double)the transverse dimension (which will in turn be greater than or equal tothat of the projection of the source). In fact, otherwise the lightscattered on the inside by a portion of diffuser would be scatteredagain by other portions before being decoupled on the outside, accordingto the known “multiple scattering” process. This process would causemixing between scattered and transmitted components of the light, andtherefore mixing of the various color components, which may becompromise the performance of the device according to the proposedobject.

“Broad-band artificial light source” is intended as any device thattransforms electrical current into luminous radiation with a visiblespectral bandwidth Δλ>100 nm, for example Δλ>170 nm, such as Δλ>250 nm,such as a white light source, or perceived as such by the eye, such asan incandescent lamp, a fluorescent lamp, a mercury vapor dischargelamp, an LED or a white light laser diode (that is, such that theprimary source is combined with a phosphor or several phosphors), or acombination of LEDs or laser diodes of different color, and the like.

The width of the visible spectral band Δλ is defined as the amplitude ofthe interval of wavelengths in the region of the visible spectrumbetween 400 and 700 nm beyond which the spectrum of the source assumes avalue below 1/e² of the peak value, where e=exp(1). It is noted that thepresence of emission peaks beyond the visible region of the spectrumdoes not contribute to the present definition of spectral width (FIG.1).

The light source of an embodiment may comprise a plurality of activeelements, identical to or different from one another, optionally groupedand/or associated with optical elements such as lenses, filters,screens, achromatic diffusers, and the like capable, for example, ofdistributing the light uniformly in a wide solid angle, or ofconcentrating it within a small and well-defined angular aperture.

“Active element” is intended as an element that emits photons, such asan incandescent filament, an ionized gas, an LED or laser diode, or anelement that absorbs photons and then re-emits them at a higherwavelength, such as a phosphor or a luminescent element, but not adiffuser, given that this latter does not emit light but simply changesthe propagation direction thereof.

“Source dimension” is intended as the minimum dimension of the largestactive element contained therein, d_(min), defined as the dimension ofthe shorter side of the rectangle that circumscribes the projection ofthis active element in a plane perpendicular to:

a) in the case of light source with anisotropic emission, the maximumemission direction (I_(max)) of said source; or

b) in the case of a light source with isotropic emission, the directiondefined by the straight line that joins the two closest points(I_(prox)) of said source and of said diffuser,

FIG. 2A illustrates the definition of minimum dimension (d_(min)) of anisotropic source 20 in the plane perpendicular to the directionI_(prox), corresponding to the plane of a nanodiffuser 26. In this caseL_(max) is around 5 d_(min).

FIG. 2B illustrates the definition of minimum dimension (d_(min)) of ananisotropic source constituted by an LED or laser diode 20 b and aphosphor 24 on a plane perpendicular to the direction I_(max).

FIG. 3 illustrates the definition of minimum dimension (d_(min)) for asource 30 of elliptical shape. In this case the minimum dimension isdefined as the shorter side of the rectangle within which the projectionof the source in a plane perpendicular to the direction I_(max) may beinscribed.

In a device according to an embodiment, source and diffuser are intendedas two separate elements, that is, the diffuser is not superimposed onan active element, or interposed between different active elements, butis positioned downstream of the active element furthest from thissource.

“Nanostructured chromatic diffuser” is intended as an object comprisingelements of a first non-liquid material transparent to visible light andhaving a refractive index n₁ dispersed in a second non-liquid materialtransparent to visible light and having a refractive index n₂, whereby|n₂/n₁−1|>0.1 and whereby the typical linear dimension, d, of thedispersed elements of the first material satisfies the condition 5nm<d<300 nm, for example being between 10 and 200 nm, such as between 50and 100 nm. In the case of a diffuser of large dimensions, an intervalof interest for the typical linear dimension, d, of the dispersedelements is 30 nm<d<50 nm. This material is capable of producingefficient separation between components with lower wavelength of theincident radiation, which are scattered, and those with higherwavelength, which are instead transmitted.

In the chromatic diffuser according to an embodiment, the dispersedelements of the first material may be:

-   -   solid nanoparticles with refractive index n₁ in the case in        which the second material is a solid matrix with refractive        index n₂<n₁.    -   gas nanobubbles with refractive index n₁ in the case in which        the second material is a solid matrix with refractive index        n₂>n₁.    -   nanovolumes of air in the case in which the second material is        constituted by a solid dendritic structure of ultra low density        silica, in turn composed of clusters of nanoparticles, which in        the presence of the aforesaid dispersion takes the name of        nanogel or aerogel.

Possible examples of chromatic diffusers of this type are silicananogels, or a dispersion of a material with a high refractive index,such as an oxide like TiO₂, ZnO, ZrO₂, BaTiO₂, Al2O₃, SiO₂, as describedin U.S. Pat. No. 6,791,259 B1, just as some of the materials listed inWO 02/089175, which are incorporated by reference in a matrix oftransparent material with a low refractive index, such as glass, plasticor polymer material such as epoxy, silicone or urea resins, or adispersion of air bubbles of nanometric dimensions in similartransparent matrices.

“Diffuser dimension” is intended as the larger size between transversedimension and longitudinal dimension, as defined above.

FIG. 4 illustrates the definition of L_(max) as “transverse dimension”of the diffuser in the case of an anisotropic emission source, 40,combined with a curved diffuser 46. In this case the maximum distanceL_(m) between two points belonging to the projection of the diffuser ina plane parallel to the direction I_(max) is less than the maximumdistance between two points belonging to the projection of the diffuserin a plane perpendicular to the direction I_(max), therefore L_(max) isdefined as “transverse dimension” of the diffuser.

A device according to an embodiment may be characterized in thatL_(max)≧5 d_(min), for example in that L_(max)≧10 d_(min). Generally, adevice according to an embodiment may be characterized in that L_(max)is in the interval between 2 and 100 d_(min).

An embodiment of a device proposed is devised (i) for indoorillumination, such as apartments, offices, warehouses, shopping malls,work environments, theatrical scenery, (ii) for outdoor illuminationsuch as roads, squares, sports grounds, parks, courtyards, (iii) forilluminating exhibited objects, such as plastics, products in displaywindows, and (iv) as separate luminous objects, for example a furnishinglamp or outdoor luminous installation.

A characteristic thereof is being able to improve the quality of thelighting that may be obtained by low energy “cold light” sources, suchas InGaN—GaN LEDs emitting in the blue region (e.g. at 430 nm) completedby the presence of a phosphor which emits broad-band radiation in theyellow region (e.g. around 580 nm). The resultant of the two colorcomponents is perceived by the eye almost as white light. However, thecorresponding “color temperature” is much higher than that of sunlight,so as to make this type of source somewhat unsuitable for indoorillumination. An embodiment allows a large part of the blue component ofthe light to be directed towards the shadows, where it is pleasing tothe eye, as it simulates the effect of skylight, directing the warmercomponent produced by the phosphor towards the areas exposed to directillumination, so as to simulate “afternoon” sunlight.

The use of nanostructured transparent materials in applications inherentto treatment of the light is a solution currently considered in agrowing number of technological spheres.

With reference to the use of the combination of artificial sources andRayleigh diffusers obtained through aqueous dispersion of silicananoparticles (SiO2), the innovative element is constituted here by theuse of solid materials.

The characteristics of the materials and of the diffusers to be producedwith these materials shall now be described in detail. The term“nanostructured diffuser material” refers in this context to anon-liquid material which exhibits spatial variations in the refractiveindex on scale lengths between 5 and 300 nm. The term “transparent”refers to the characteristics of the absorption spectrum of thematerial. The depth L of the material, in the direction of lightpropagation, depends on the type of application, and may vary from a fewmicrons (for applications on optical microcomponents) to tens of meters(in the case of large installations).

The main characteristics which describe the properties of the materialare the typical dimension of the nanostructure (or the dimensions if, asin the case of nanogels, fluctuations have different scale lengths) andthe concentration, that is, the number of fluctuations per unit ofvolume. Hereunder, the material is described in terms of a homogeneousmatrix and of a dispersion of nanoparticles with diameter d, intendingin this case also to include nanogels.

Dimensions of the nanostructure. These allow, where necessary, efficientscattering in order to obtain the desired ratio between scattered andtransmitted light, and therefore, for example, between luminosity of theareas “in shadow” and of the areas “in the sun”, using minimum depths ofdiffuser. This efficiency is obtained maintaining the scattering processas much as possible in Rayleigh regime, in order to obtain the maximumcolor contrast or, more precisely, the maximum variation in CalibratedColor Temperature (CCT) between scattered and transmitted light. Thisimplies, strictly speaking, the choice of nanoparticles with a diameterof less than 50 nm, that is, d<λ/10. In fact, when the diameterincreases, there is a considerable decrease in the value of the ratiobetween scattering amplitude in the blue region (λ=450 nm) and, forexample, in the red region (λ=630 nm). Considering, for example, thecase of a dispersion of nanoparticles with refractive index n₁=2.7 in auniform material with index n₂=1.5, it is obtained that in the regime ofnegligible multiple scattering, more specifically in the limit of a verysmall number of particles, τ₄₅₀/τ₆₃₀=3.86, 3.95, 3.68, 1.56, 0.63 fornanoparticle diameters d=20, 50, 100, 200, and 500 nm, respectively,where τ_(λ) is defined as the scattered fraction of light at a givenwavelength λ [H. C. van de Hulst, “Light Scattering by Small Particles”,Dover Publications, New York 1981, which is incorporated by reference].Moreover, as the diameter increases, the scattering efficiency becomesanisotropic, increasing at small angles (forward scattering). However,as it increases with the 6th power of the particle diameter, τ∝d⁶, giventhe same numerical concentration, it is evident that large diametersmake the scattering process much more efficient. It is thereforenecessary to evaluate, case by case, the optimum compromise betweenefficiency and color contrast desired. Angle and frequency resolvedmeasurements and numerical simulations show that for linear dimensionsof the dispersed elements up to d≈100 nm, scattering varies little fromthe ideal Rayleigh regime. Values d≈200 nm instead imply a considerablevariation. However, dispersed elements of these dimensions may be usefulif wishing to simulate the effect of a sky with slight mist, that is,which scatters at small angles (i.e. from areas close to the sun) lightwith a certain whiteness. With regard to the minimum acceptable valuesfor the linear dimension of the dispersed elements, it must be mentionedthat in the literature values d<20 nm are already considered such as tomake the scattering phenomenon almost negligible, for the materialdepths typically considered. For example, in US 2008/0012032, which isincorporated by reference where the use of a dispersion of nanoparticlesin a transparent medium is considered in order to vary the average valueof the refractive index of the composite material, the requirement toprevent scattering translates into a requirement on the diameter of thenanoparticles d<λ/20, where λ is the wavelength of the incidentradiation. Considering the fact that the decrease in scatteringefficiency in relation to the decrease in the linear dimension of thedispersed elements may be compensated by the increase in theconcentration of these elements and/or in the depth of the diffuser, itis established that the linear dimension of the dispersed elements, d,useful for practical purposes for an embodiment is within the interval 5nm<d<300 nm, bearing in mind that an ideal interval is 50 nm<d<100 nm.

Concentrations and depths. The second quantity that determinesscattering efficiency is the concentration of nanoparticles, thescattered fraction of light being proportional to the number ofparticles per unit of volume, T∝n, for a fixed sample length and in thelimit of low concentration. There are two factors which principallylimit the maximum concentration value usable. The first is a conceptuallimit, due to the fact that when the particles are at an averagedistance from one another of a few diameters, a fact that occurs forconcentrations of over a few %, they start to organize themselves with ashort range spatial order, giving rise to effects of interference whichdisturb the scattering process. A second limit, of practical type,relates to the possible occurrence of the phenomenon of clustering ofthe nanoparticles at high concentrations, due to the presence of a shortrange attractive potential. Owing to the dependence T∝d⁶, even a minimumpercentage of clustered particles (large effective d) is capable ofcreating a dominating effect on the scattering process. In the case ofmaterials with depths of a few mm, the concentrations required to obtainthe desired scattering values are nonetheless very low. Considering, forexample, the case of particles with refractive index n₁=2.7 in atransparent medium with depth L=5 mm and with refractive index n₂=1.49,to obtained scattering efficiency η=0.5 in the case of diameters d=20,50, and 100 nm numerical concentrations of n=4.7286*10¹⁴, 1.9368*10¹²,3.0263*10¹⁰ particles per cm³ may be necessary, which correspond to afraction of volume of 0.1981%, 0.0127%, 0.0016%, respectively. Ifinstead high concentrations are to be used, for example of 1% in volume,which may require high control of the manufacturing process of thediffuser material in order to avoid clustering, the same efficiencyvalue (η=0.5), may be obtained for the same diameters (d=20, 50 e 100nm), using diffuser depths up to only a few microns, i.e. 990 μm, 63 μm,and 7.92 μm respectively. This indicates the possibility of producingthe technology described, in certain regimes, also in thin films.

Absorption. The material absorbs a minimum part of the total incidentradiation, in order to maintain maximum efficiency of the device interms of energy consumption. Moreover, it does not exhibit selectiveabsorption in any particular region of the visible spectrum, in ordernot to introduce unnatural and, therefore, undesirable color effects.For example, selective absorption in the blue region may reducecoloration of the shadows, while selective absorption in the red regionmay make the color of light in directly illuminated areas less “warm”.

Reflection. Reflection of the light of the source on the entry and exitface of the diffuser may represent a drawback in relation to the energyefficiency of the device. In the case of a matrix with index n₂=1.5,this amounts to around 8% for normal incidence, but the value increasesconsiderably in relation to increase in the angle of incidence, for nonpolarized light. It is noted that the problem also exists for nanogels,despite the fact that for these n₁=1, as due to their fragility they mayrequire to be contained in a transparent element. As well as the problemof efficiency, the problem of the quality of the illumination producedis also considered. In fact, in nature the pureness of the colors of thesky and the beauty of the color contrasts between light and shadow areclosely linked to the presence of a dark background (interstellar space)behind the sky. Indoors, it may be, therefore, essential to prevent thelight of the source reflected by the diffuser from illuminating walls(e.g. ceilings), from which it would then be redirected onto the scene.In an embodiment the problem of reflection, in cases in which it exists,may be solved either by introducing light absorbing screens or, whereefficiency of the device is a crucial parameter, using appropriateanti-reflective treatments.

A sector of particular interest is the “aerogel or nanogel” sector. Inspite of their name, these are solid materials, dry (with no liquidcontent), spongy and porous, typically obtained through supercriticalevaporation in autoclaves of the liquid component of a gel (or of aliquid substituted for this after formation of the gel). Unlikeconventional evaporation, this process maintains unchanged the structureof the solid component of the gel, which forms a rigid foam withultra-low density. In fact, over 99.8% of the volume of the finalmaterial may be occupied by air. The material thus produced presents amicroscopic dendritic structure, with fractal characteristics, formed bynanoparticles with typical diameter of 2-5 nm, grouped in clustersspaced apart from one another by voids, said voids which may haveaverage dimensions of less than 100 nm. The first aerogels to bedeveloped, and which are still the most widely used, are those obtainedfrom silica gel (SiO₂). However, aerogels obtained from other materialsexist, such as aluminum oxide, chromium oxide, tin dioxide, carbonoxide, etc. In spite of the extremely low mass, the material presentsconsiderable characteristics of rigidity, so much so that it is used inspace technologies, for sporting equipment, etc. Limiting analysis tosilica aerogels, of greater interest for the lighting applicationsinherent to an embodiment, it is transparent (that is, it absorbs aminimum quantity of light), and scatters part of the blue component byRayleigh scattering. In fact, the nanostructure of which it isconstituted has two typical length scales: that of the clusters ofnanoparticles and that of the voids between these clusters, which mayboth be well below the wavelength of the light. For this reason aerogelsare also called “solid smoke” in jargon. Due to their transparency,optimal heat insulation and also their capacity to scatter light, silicaaerogels are today also used in building, to produce rooflights andwindows capable of blocking heat from sunlight, or of retaining heat inthe building, and at the same time of very effectively distributing thesunlight that hits them on the inside. For these applications aerogelgranules are principally used, that is, fragments a few millimetersthick, contained inside windows of transparent material. The useillustrated may be capable of producing a blue coloration of theshadows. With reference to an embodiment proposed here, it may beimportant to observe that the use of aerogels for illumination withartificial light has not been considered previously.

As it has been stated above, the illumination device of an embodimentproposes an object contrary to the typical object of prior artillumination devices, that is, to make a source (which is typically, butnot necessarily, chromatically uniform) chromatically non-uniform, witha type of non-uniformity (scattered blue and transmitted yellow)converse to that generated “spontaneously” by fluorescent LEDs. Acharacteristic of a device of an embodiment is to provide a nanodiffusermuch larger than the last active element (for example phosphor), andposition it downstream of this active element, for example separatedtherefrom. This difference in dimensions, positioning and spatialseparation between source and diffuser are the factors that break the“spherical” symmetry of the system, with the aim of uniforming angulardistribution of the colors, so as to allow separation of the twoscattered (blue) and transmitted (yellow) color components, as can beunderstood from the descriptions of the different embodimentsillustrated hereunder.

Embodiments of the Illumination Device in “Midday” Configuration

A first example of device according to an embodiment is illustrated inFIG. 5A, where an anisotropic source of white light 52 emits light ofuniform spectrum over a wide solid angle, illuminating the nanodiffuserelement 56 subtending this angle. In the presence of the nanodiffuser,the yellow component 57 is scattered less than the blue component 55.The result from the viewpoint of method of illuminating an object on thescene is illustrated in FIGS. 5B-D. In FIG. 5B an object 53 isilluminated both by direct light (yellow) and by the light scattered bythe panel (blue); in FIG. 5C an object 53 c is illuminated principallyby direct light (yellow), in this case being provided with a screen 58which removes the majority of the light coming from the diffuser panel56, while object 53 cc is illuminated only by the light back diffused(blue) from the diffuser panel 56; finally, in FIG. 5D an object 53 d isilluminated principally by the scattered light (blue), as the directlight is removed by a screen 58 d which generates shadow on this object.

The effect in the case of illumination of an environment by means of asource 52 e combined with a diffuser 56 e is shown in FIG. 5E. Here thescene is partly illuminated by the sum of direct and scattered light 51and partly only by scattered light 59. It is noted that the areailluminated by scattered light 59 comprises the shadow of an object 53 elocated in the center of the scene. It also comprises an outer areawhich would not be illuminated by the light cone produced by the sourcein the absence of the diffuser element.

A second example of device is shown in FIG. 6, where an anisotropicsource of white light is constituted by a blue LED or laser diode 60associated with a phosphor 64 which emits in the yellow region. Thesource is incorporated in a transparent containing body 62, where thenanodiffuser 66 is incorporated in a collecting lens 63, forming thelower part of the transparent body 62. The lens 63 has the object ofcollimating the radiation generated by the source constituted by an LEDor laser diode 60 and a phosphor 64, or of reducing the angle ofdivergence thereof. In the conventional configuration, i.e. in theabsence of the nanodiffuser 66, the lens 63 would collimate the whitelight produced by the source, creating a cone of given angulardivergence and of given transition gradient between light and dark.Conversely, the device 72 according to an embodiment, as shown in FIG.7, produces a narrow inner cone of yellow light 71 and a wider cone ofblue light 75.

In a third embodiment shown in FIG. 8, two technical modifications areintroduced, also separately, in order to improve the efficiency of thedevice. In the first the optical collimation element, i.e. the lens 83,is positioned at a certain distance from the source and separatetherefrom. In this case, this lens, which contains the nanodiffuser 86,is provided with an anti-reflective treatment. An object of thistreatment is to optimize transmission of the “warm” component of theradiation emitted by a source constituted, for example, by an LED orlaser diode 80 and a phosphor 84, preventing reflections which couldreduce the efficiency of the device and direct part of this component tothe outer area, reducing the contrast. An object of the reflector is toretrieve the “cold” component back-scattered by the nanodiffuserelement. In fact, in Rayleigh scattering regime, the scatteringefficiency is identical in the two half-spheres which comprise thedirection from which the light comes, and towards which the lightpropagates, which implies that a quantity of scattered light identicalto that directed on the scene is directed back towards the lamp. One wayof solving, or at least considerably reducing the problem is that ofinserting the lens and the source inside a reflector 87 (whetherspherical, parabolic or of different shape, with a smooth or corrugatedsurface) which re-directs the back-scattered light outward. As thescattered light is generated by an extended element and in randomdirections, the presence of a reflector in the vicinity of the diffuserdoes not greatly alter the divergence of the radiation coupled therebyon the outside. In the case in which the geometry of the system demandsa considerable distance between lens and reflector, the use ofcorrugated surfaces for the reflecting walls ensures maintenance of thedesired divergence in the scattered light.

A schematic example of the type of illumination obtainable with anembodiment of a device proposed here may be understood from the diagramspresented in FIGS. 9 and 10. In the first case, a conventional spotlight90 illuminates with a defined cone of white light 91 a particular regionof an environment, leaving the rest in the dark. In the second case, aspotlight 100 according to an embodiment, for example of the type shownin FIG. 8, generates a “warmer” light cone 101 than the cone of “white”light of the preceding figure, of the same aperture, having removedtherefrom the “colder” component, which is distributed in the outer cone103. The outer regions of the environment, which were previously in thedark, are instead now exposed to scattered blue light, as occurs in thecase of shadows in outdoor environments on Earth. It may be important toobserve that the terms “blue” and “yellow”, or “cold” and “hot” referredto light, are only indicative, and used in order to symbolicallyindicate a concept. The quantitative parameter that determines thecharacteristics and therefore the “quality” of the device is thepreviously described “calibrated color temperature” (CCT). Measurementstaken using a nanodiffuser of nanogel material with a depth of 12 mm anda blue LED with yellow phosphor with CCT=5296K, produced for thetransmitted “warm” component a value of CCT=4334K, corresponding to atypical value for direct sunlight in the early afternoon, and for thescattered “cold” component a value of 9433K. The results are shown inFIG. 16.

Measurements taken using a nanodiffuser of PMMA/TiO₂ material with adepth of 12 mm and a blue LED with yellow phosphor with CCT=5296K,produced for the transmitted “warm” component a value of CCT=4417.9K,corresponding to a typical value for direct sunlight in the earlyafternoon, and for the scattered “cold” component a value of 7447.2K.The results are shown in FIG. 17

From the data in the FIGS. 16 and 17 it is evident that the decrease ofthe blue peak associated with the LED which is obtained in thetransmitted component as a result of Rayleigh scattering makes thespectrum of this component, which is the one that illuminates the scenedirectly, much more similar to the bell-shaped profile of the black bodyspectrum, thus making it more similar to that produced by directsunlight.

In a fourth embodiment of FIG. 11 a plurality of light sources wereused, designated with 110, positioned downstream of which is ananodiffuser 116.

The type of device shown in FIGS. 6 and 8 is not limited to the use ofLEDs or laser diodes, but may make use of any type of light sourcecharacterized by a sufficiently broad spectral bandwidth. The device issuitable for indoor illumination, such as hotel rooms, or study areas inapartments, etc., that is, all those environments in which lighting ofonly one area is preferred, or to create a scenographic effect, or toreduce consumption. A second area of interest is that of roadillumination, which would benefit from the possibility of preferentiallyilluminating an area in direct light (for example the road), with aspectral component (for example yellow) where the eye is particularlysensitive, maintaining a weaker and more scattered illumination in thesurrounding areas. All this with the natural coloration of sunlight andskylight.

Embodiments of the Illumination Device in “Sunset” Configuration

An example of device according to an embodiment in “sunset”configuration is shown in FIGS. 12A and 12B, where a white light source120, constituted by a plurality of white light LEDs or laser diodes (forexample, UV LEDs or laser diodes combined with phosphors of differentcolors, or combinations of LEDs or laser diodes which emit in the red,in the green and in the blue region) of the type shown in FIG. 11, isschematically produced in the shape of an elongated parallelepiped. FIG.12A shows the source 120 in a side elevation, while FIG. 12B shows thesource 120 in a top plan view. The source 120 is contained in atransparent structure on the side 120 a, from which the radiationemitted is directed towards a nanodiffuser 126. The source,appropriately collimated in the horizontal plane, sends the broad-bandlight produced thereby inside the nanodiffuser 126. This diffuser isconfigured as a panel whose length in the direction of propagation oflight is much greater than its depth. The height of the nanodiffuserpanel may have different values according to different needs. In orderto allow the light transmitted to propagate inside the entire length ofthe panel, in spite of the fact that it may be very thin, it may beimportant for it to act as a waveguide, that is, not to coupletransmitted light on the outside (or to couple a minimum part thereof)in the absence of the dispersed elements. In other terms, the diffuserelement without the nanostructure is such that it does not couple lighton the outside. A method of achieving this objective is to use, as thediffuser material, a dispersion of nanoparticles in transparentvitreous, plastic or polymer material, whose refractive index n₂≈1.5,being greater than the refraction index of air (n_(air)≈1), provides alimit angle for internal reflection, with respect to the normal of thesurface, of θ_(lim)=42 deg. For this reason, the panel of transparentmaterial without nanoparticles incorporated acts as an excellentwaveguide, capable of transmitting from one part to the other the lightof the source by total internal reflection, as shown by the lines 122and 123. In the presence of nanoparticles, the light scattered therebyat −42 deg<θ_(scat)<42 deg is decoupled from the guide, so that itbecomes visible to the observer, as shown in FIG. 12B. The lightscattered at greater angles is instead coupled, to then be scattered ifnecessary in a subsequent scattering event. In this manner, the portionof diffuser closest to the source scatters almost all the bluecomponent; the subsequent portion will prevalently scatter the green, asthis is the remaining component with the shortest wavelength, and willscatter it with a slightly lower scattering efficiency (given thegreater wavelength) than that characterizing scattering of the blue inthe previous region. This implies a decrease of luminosity progressivelyalong the entire diffuser, until only the red radiation componentremains, to be scattered in the end portion of the diffuser.

In a variant of this embodiment, shown in FIG. 12C, objects 127 may beincorporated in the nanodiffuser panel 126, for example white objects,capable of locally scattering all the components of incident light, andtherefore simulating clouds. In another variant, panels of differentlength, or characterized by different scattering coefficients, may beplaced side by side in the installation (in the direction of depth ofthe diffuser, i.e. in the direction of observation), in order tosimulate the various scattering processes that take place at differentheights in the atmosphere. This allows reconstruction, for example, ofthe fact that the low part of the atmosphere at sunset is invested bythe orange or even red component of solar radiation, as all the othercomponents have been scattered or refracted, while the high part of theatmosphere remains illuminated by the white light of the sun, andtherefore scatters blue colored light towards us.

A further embodiment of an illumination device in “sunset” configurationis shown in vertical elevation in FIG. 13A and in a bottom plan view inFIG. 13B. In this embodiment, the nanostructured diffuser element 136has the shape of a cylinder. The operating principle is the same as theprevious case: a source 130 illuminates the cylinder from below (or,equivalently, from above) and the light, which propagates guided alongthe axis of the cylinder, scatters all the color components, startingfrom blue through to red. This device is intended as an indoor lamp, or,in the case of considerable dimensions, as an outdoor installation.

A further embodiment of the lighting device, susceptible of operatingboth in “midday” and in “sunset” configuration, is shown in FIGS. 14A,14B, 14C, 14D, 14E and 14F. FIG. 14A shows in vertical elevation a lightsource 140 which illuminates from below a cylinder 146 of nanostructuredtransparent material similar to the previous one but perforated inside,as shown in FIG. 14B. The “solid” portion of the nanostructured cylinder146 once again acts as a waveguide, if it is illuminated by a sourceplaced at one end and which has the shape of a ring, as shown in FIG.14C (viewed from above). In this operating mode the device acts to theview in an analogous manner to the device in FIG. 13, setting the scenewith the colors of the sky at sunset, as shown in FIG. 14A.

Unlike the previous case, FIG. 14D shows a light source constituted by alamp of elongated tubular shape 140 a, or a white diffuser cylinderilluminated from below, capable of generating white light whichpropagates outwards, designed to be inserted in the hole of thenanostructured diffuser 146, as shown in FIG. 14E, whose light istherefore also scattered by the hollow nanostructured diffuser cylinder.In the case of this second type of illumination, an observer positionedin front of the cylinder will see the inner luminous tube, which emitswhite light, without the scattered blue component. This luminous tubewill therefore appear yellow, i.e. the color of the sun, as shown inFIG. 14F. With regard to the lateral parts of the cylinder, the observerwill see the blue component of light generated by the central tubescattered towards him, that is, he will see the color and light of thesky. It is interesting to note that the observer will see the samedistribution of colors regardless of his position with respect to thesource, that is, from all sides. This fact will allow him to discoverthat the different colors are not linked to different colorations of theobject, but are inherent to the difference existing between transmittedand scattered light. Other possible variants of the present embodimentprovide for a structure with a square, rectangular or elliptical base.In this case, the lack of symmetry creates in the colors different tonesaccording to the different visual angles. The two light sources 140 and140 a will be used alternatively to each other. Switching off the firstsource 140 and switching on the second 140 a, the transition is madefrom the condition of sky at sunset to the condition of sky and sun atmidday. The spectacular transition between daytime and evening scenerywill reinforce the proof of the role played by the scattering processesin determining the color of light.

With regard to coupling of the light produced by the source with thepanel in the embodiments with “sunset” configuration, it will beproduced by installing appropriate optical elements, in the case inwhich the source or sources used do not have the desired angulardivergence. A characteristic for optimizing coupling is that the sourceproduces sufficiently low divergence in the direction of depth of thediffuser panel to allow guided propagation of the light in the material.The source, for example composed of a battery of LEDs or laser diodes,will also be extended in vertical direction, in the case of the devicein FIG. 12A, in order to uniformly illuminate the entire extension ofthe panel 126, or circular, in the case of the device in FIG. 14A inorder to illuminate the base of the tubular diffuser cylinder 146. Withregard to the spectral characteristics of the source, it is noted that asource which is white to the eye, as it excites in an adequatelybalanced manner the three types of detectors (i.e. R. G. B), may belacking important portions of the spectrum, in relation to that producedby a black body at the same CCT. This source may prove unsuitable forthe purpose, as it is incapable of generating scattered light of theexpected color in specific spatial portions of the diffuser, which wouldbe dark. If we consider, for example, the case of a blue LED withphosphor emitting in the yellow region, it is evident that this sourcewill not be able to create the bright red which is expected toilluminate the end part of the diffuser, so as to simulate sunlightilluminating the clouds at sunset, simply because the source does nothave that spectral component. Ideal sources are therefore either thosewith black body spectrum, or those produced by appropriate combinationof LEDs or laser diodes and phosphors of different colors. It is notedthat, in the case of a combination of different sources, these do notnecessarily require to be integrated in a single element, but may besimply positioned side by side, provided that this positioning issufficiently compact not to be perceived as heterogeneity of colorsscattered in the vertical plane of the diffuser.

A device according to an embodiment may be used as luminous furnishingobject, capable of setting the scene with the beauty of the colors ofthe sky. In this case it is used as object to be observed, rather thanas a source that illuminates a scene. However, it is possible to produceboth functions in the case of a panel, or system of panels, of largedimensions, which cover, for example, the entire ceiling, or an entirewall of a room, or even several walls. In this case, the device mayreplace conventional illumination, providing, just as the sky,illumination scattered throughout the environment. An importantadvantage with respect to the type of illumination proposed in theprevious embodiments is that in this case it is unnecessary to place anysource behind the panel, at a certain distance, a fact that considerablyreduced the dimensions.

Embodiments of the Illumination Device in “After Sunset” Configuration

An example of device according to an embodiment the invention in “aftersunset” configuration is shown in FIG. 18 where two white light sources180, positioned at a height of ⅔ with respect to the total height fromthe floor to the ceiling, emit light of uniform spectrum at a certainangle towards the ceiling, illuminating the nanodiffuser element 186.Part of the light is back diffused by the chromatic diffuser 186 anduniformly illuminates the scene in a blue color. The light which passesthrough the diffuser element 186 is absorbed by a black panel 182positioned behind element 186.

In a further embodiment, the light source is totally separated andspaced apart from the chromatic diffuser. In this case, the term“device” is to be meant as the combination of a light source and achromatic diffuser, irrespective of whether they are structurallyintegrated or not. An example of such embodiment is schematically shownin FIG. 15, in which the chromatic diffuser is a panel 156 integrated ina wall of a building 158, and the light source is integrated within anexternal lamp 150, for example a street lamp. The chromatic diffuser maybe incorporated in the structure of a window, and actually replace theglass(es) of a window. In such embodiment the space inside the buildingis illuminated according to the principles described above. In anotherexample of such embodiment the chromatic diffuser is a panel 156 similarto the one in FIG. 15 integrated in a wall of a building 158, and thelight source is integrated within a lamp positioned inside the building.In this case the space inside the building is illuminated by the lightback scattered from the panel 156.

Some embodiments have been described, but it is evidently susceptible tonumerous modifications and variants within the scope of the disclosure.

From the foregoing it will be appreciated that, although specificembodiments have been described herein for purposes of illustration,various modifications may be made without deviating from the spirit andscope of the disclosure. Furthermore, where an alternative is disclosedfor a particular embodiment, this alternative may also apply to otherembodiments even if not specifically stated.

The invention claimed is:
 1. An illumination device comprising: abroad-band artificial light source comprising one or more activeelements which are configured to emit photons or to absorb photons andthen re-emit photons at a higher wavelength; and a chromatic diffuser;wherein the maximum dimension of said chromatic diffuser is equal to orgreater than twice the minimum dimension of the projection of thelargest of said active elements in a plane perpendicular to the maximumemission direction of said light source, wherein the maximum dimensionof said chromatic diffuser is defined as the maximum distance betweenany two points belonging to the projection of the chromatic diffuser ina plane perpendicular to the maximum emission direction; and wherein thechromatic diffuser is configured to produce a narrow inner cone oftransmitted light and a wider outer cone of diffused light, thetransmitted light being more intense and warmer than the diffused light.2. The illumination device according to claim 1, wherein the broad-bandartificial light source is formed by a blue or ultraviolet LED or laserdiode, associated with a phosphor which is configured to emit at leastin the yellow region.
 3. The illumination device according to claim 1,further comprising a lens configured to collimate the transmitted light.4. The illumination device according to claim 1, further comprising alens configured to collimate the radiation generated by the broad-bandartificial light source so as to form the inner cone, or to reduce theangle of divergence of said radiation.
 5. The illumination deviceaccording to claim 2, further comprising a lens configured to collimatethe radiation generated by the broad-band artificial light source so asto form the inner cone, or to reduce the angle of divergence of saidradiation.
 6. The illumination device according to claim 1, furthercomprising a plurality of light sources.
 7. The illumination deviceaccording to claim 1, wherein the broad-band artificial light source isformed by a plurality of white light LEDs.
 8. The illumination deviceaccording to claim 1, comprising an array of blue or ultraviolet LEDs,an array of phosphors and an array of collimating lenses, each LED beingoptically coupled to a corresponding phosphor, each phosphor beingcoupled to a corresponding lens.
 9. The illumination device according toclaim 1, wherein said broad-band artificial light source comprises aplurality of lenses, each lens being optically coupled to acorresponding active element, said lenses being interposed between theactive elements and the chromatic diffuser.
 10. An illumination devicecomprising: a broad-band artificial light source comprising one or moreactive elements which are configured to emit photons or to absorbphotons and then re-emit photons at a higher wavelength; and a chromaticdiffuser; wherein the maximum dimension of said chromatic diffuser isequal to or greater than twice the minimum dimension of the projectionof the largest of said active elements in a plane perpendicular to themaximum emission direction of said light source, wherein the maximumdimension of said chromatic diffuser is defined as the maximum distancebetween any two points belonging to the projection of the chromaticdiffuser in a plane perpendicular to the maximum emission direction; andwherein the chromatic diffuser is configured to produce, at each pointof it, a corresponding narrow inner cone of transmitted light and acorresponding wider outer cone of diffused light, the transmitted lightbeing more intense and warmer than the diffused light.